The present invention concerns a pin tumbler cylinder lock which is more resistant to picking than the cylinder locks of this type known heretofore.
Cylinder locks generally comprise a number of pin tumblers spring loaded within bores which must be depressed by the proper projections in the key in order for the plug of the lock to be rotated within the shield.
The bores in the plug and shell, due to manufacturing limitations, always vary in diameter. In cylinder locks the holes are drilled. The tool for drill bits doing the work is quite long in relation to its diameter. Also because of the flexibility of the drill and uneven consistencies in the metal being drilled, and because they are not perfectly formed at the cutting edges, these drills usually drill holes of varying diameters. Drills will also run off of a true center line. These manufacturing limitations cause the whole inner plug and shell never to be matching exactly, even in the most expensive and well made locks. Because of these manufacturing deficiencies, lock picking is made easier because once the end of the pin tumbler is aligned with the shear line, a shim can be introduced to maintain the separation of plug tumbler pins and shell tumbler pins. As the next pair of tumbler pins are picked, the shim can be advanced linearly to continue and maintain the separation between the plug tumblers and shell tumblers, and so on until all of the tumblers are separated along the shear line with the shim maintaining this separation. Locks of this type can also be picked without the use of a shim as follows. Once the end of the pin tumbler is aligned with the shear line, its position can be held by applying a slight turning pressure on the plug. As the next pair of tumbler pins is picked, slight further twisting of the plug maintains the ends of the second pin tumblers aligned with the shear line, and so on until all the tumblers are separated at the shear line and the plug can be rotated in the shell. This is made possible by the loose fit of the pin tumblers in the bores and by the non-mating surfaces of core and shell pin tumblers which is a direct result of having a linear shear line.
This problem is more acute with dual pin tumbler locks, since the bores and pin tumblers in such locks are of a larger diameter than in single pin tumbler locks. Such a dual pin tumbler lock was disclosed in our Pat. No. 50,984.